In general, a reference value of the amount of fuel fed to an engine with respect to the amount of air taken into the engine is determined based on an engine speed, a throttle opening degree, an air-intake pressure, etc. The reference value indicates a suitable amount of fuel set for a constant speed running of the engine. A controller is configured to control an operation of the engine and to determine the fuel feed amount set as the reference value based on the engine speed, the throttle opening degree, etc. received from sensors. An injector feeds the determined amount of fuel to air in the engine. However, during a deceleration state of the engine, the amount of fuel to be combusted correctly tends to be less than the reference value. For this reason, the controller determines whether or not the engine is decelerating based on the engine speed and the throttle opening degree and executes a control (fuel amount reduction control) to reduce the fuel feed amount to less than the reference value if it is determined that the engine is decelerating (e.g., Japanese Laid-Open Patent Application Publication No. Hei. 10-184423).
A specific control method will now be described. In a graph with an engine speed (N) on a horizontal axis and a throttle opening degree (Th) on a vertical axis, the relationship between the throttle opening degree and the engine speed under a steady state, corresponding to a predetermined gear position, are represented by a line (reference curve) that rises to the right. The graph indicates that the engine is decelerating when the throttle opening degree corresponding to the engine speed at a moment is below the reference curve and the engine is accelerating when the throttle opening degree is above the reference curve.
It is necessary to determine whether or not the engine is decelerating while considering an error contained in a value of the throttle opening degree, etc., detected by a sensor. Typically, a threshold curve is set below the reference curve so as to conform to the reference curve. When the detected throttle opening degree is below the threshold curve, the controller determines that the engine is decelerating, and executes the fuel amount reduction control to reduce the fuel feed amount to less than the reference value.
With respect to the relationship between an air-intake pressure and the engine speed under the steady state, a reference curve, and a threshold curve are set in the same manner as described above. A controller compares a detected value of the air-intake pressure corresponding to a detected value of the engine speed from sensors to the threshold curve to determine whether or not the engine is decelerating and executes the fuel amount reduction control.
It is difficult to determine whether or not the engine is decelerating in a low engine speed range based on the throttle opening degree and the engine speed. This is because the reference curve and the threshold curve set as described above are present in close proximity to each other in a vertical direction in the low engine speed range so that the controller cannot in some cases correctly determine whether or not the engine is decelerating unless the throttle opening degree is detected precisely.
Even in medium and high engine speed ranges, it is in some cases difficult to correctly determine whether or not the engine is decelerating, based on the throttle opening degree and the engine speed of the engine. For example, when an operator performs a quick throttle operation to decrease the throttle opening degree during, for example, an accelerating state of the engine, it takes some time for the engine to transition from the acceleration state to the deceleration state through a constant speed state. As a result, the controller may determine that the engine is decelerating based on the relationship between the throttle opening degree and the engine speed, although the engine is still accelerating. On the other hand, when the operator performs a quick throttle operation to increase the throttle opening degree during a deceleration state of the engine, the controller may determine that the engine is accelerating although the engine is still decelerating.
The same problems arise in various engines irrespective of the type of engine which varies in the number of cylinders, arrangement of the cylinders, etc., and the type of mobile objects such as motorcycles. In addition, the same problems arise when it is determined whether or not the engine is decelerating based on the air-intake pressure and the engine speed.